Typically a power supply in a desktop computer requires the maintenance of an auxiliary output voltage for the operation of the service processor and for internal house keeping. This auxiliary output voltage is generally a separate small power supply within a power supply and is used to provide continuous current even when other power outputs are turned off However, this small power supply adds extra cost and instrumentation to the overall system.
FIG. 1 depicts a conventional auxiliary bias circuit in a power supply. The area enclosed by the dashed line contains a flyback auxiliary power supply 10. The remaining represents a boost circuit and a chopper circuit. The boost circuit and chopper circuit include a boost inductor 12, MOSFET transistors 16, 24, a pulse width modulator 18, capacitors 20, 44, a transformer 22, an inductor 42 and diodes 14, 38, 40. The flyback auxiliary power supply 10 includes a pulse width modulator 26, a transistor 28, a transformer 30, a diode 32, a capacitor 33, an error amplifier 34 and an opto-coupler 36.
To further understand the operation of a conventional auxiliary bias circuit in a power supply, please refer now to the flowchart of FIG. 2. First, the input voltage to the flyback 10 is provided by the boost voltage developed across the bulk capacitor 20, via step 102. Next, the pulse width modulator 26 is used to chop this voltage through transformer 30 and transistor 28 at a high switching frequency (100 kHz), via step 104. Transformer 30 then transforms the bulk voltage to the secondary side where it gets rectified by diode 32 and filtered by capacitor 33, via step 106. Finally, the error amplifier 34 regulates the output voltage by modulating the pulse width of pulse width modulator 26, via step 108.
Because the flyback auxiliary power supply 10 is segregated from the boost circuit, it requires a separate pulse width modulator 26, a separate transformer 30, and a separate MOSFET transistor 28 which increases the cost of the power supply system.
Accordingly, what is needed is a circuit that decreases the cost of the power supply system. The present invention addresses such a need.